This invention relates to an improved oxidation catalyst, its method of preparation, and its use in a process for the preparation of carboxylic acid anhydrides from hydrocarbons. More particularly, it relates to a novel and simpler method for the production of vanadium-phosphorus-oxygen catalyst composites providing increased yields. Stil more particularly, it relates to the production of maleic anhydride from n-butane or n-butene, in a vapor phase process employing the foregoing catalyst composition.
Methods for the preparation of catalyst compositions of vanadium, phosphorus, and oxygen, and the use of these compositions as catalysts in hydrocarbon oxidations are known in the art.
Such preparative methods can be generally categorized as being aqueous-based or organic-based and employ either homogeneous solutions or heterogeneous mixtures (e.g., suspensions) of at least one of the components (e.g., a vanadium containing compound) which eventually forms the catalyst composition.
The particular method of preparation selected will depend on the various combination of properties sought to be imparted to the catalyst and the commercial attractiveness of the process. Particularly significant properties sought to be influenced by the catalyst preparative methods of the prior art include the vanadium valence, the P:V atomic ratio, the crystal phases of the catalyst, and the surface area of the catalyst.
While at least one patent seeks to impart a vanadium valence of less than +3.9, namely, U.S. Pat. No. 4,178,298, a majority of patents seek to obtain a vanadium valence between +4 and +5.
One preferred way of achieving this is to begin with vanadium in the +5 valence state and reduce the valency to less than +5 or alternatively to start with a vanadium compound having a valency of less than +5. A wide variety of reducing agents can be employed for the former reducing method approach. Representative of such reducing agents include acids such as hydrochloric, hydriodic, hydrobromic, acetic, oxalic, malic, citric, formic and mixtures thereof such as a mixture of hydrochloric and oxalic may be used. Sulphur dioxide may be used. Less desirably, sulfuric and hydrofluoric acids may be employed. Other reducing agents which may be employed are organic aldehydes such as formaldehyde and acetaldehyde; alcohols such as pentaerythritol, diacetone alcohol and diethanol amine. Additional reducing agents include hydroxyl amines, hydrazine, and nitric oxide or nitric acid.
Reducing methods also can be classified according to whether the vanadium compound is dissolved, e.g., solution reducing methods, or not, e.g., heterogeneous reducing methods.
In accordance with solution reducing methods, a vanadium compound having a valence of +5 such as V.sub.2 O.sub.5 is dissolved in a solution containing the reducing agent. Because many strong acid reducing agents, such as HCl, also function to dissolve the vanadium compound and, therefore, act as a solvent, the solvent and reducing agent can be the same (see for example Kerr, U.S. Pat. No. 3,288,721). Thus, a strong acid reducing agent (e.g., HCl) can be employed in an aqueous or non-aqueous (e.g., organic) medium to achieve dissolution and reduction therein. For example, Bergman et al., U.S. Pat. No. 3,293,268 discloses an aqueous solution reduction process for preparing a V-P-O containing catalyst wherein V.sub.2 O.sub.5 and phosphoric acid are dissolved and reacted in a concentrated aqueous solution of a hydrogen halide, e.g., HCl, and the resulting reaction product is heated to 300.degree. to 500.degree. C. to yield a catalyst having a P:V atomic ratio of 1.02:1 to 1.5:1. This patent also discloses that when n-butane is admixed with an oxygen containing gas in the presence of 3 to 50 moles of steam per mole of n-butane, the yield of maleic anhydride using the catalyst described therein is improved. However, the weight % yield of maleic anhydride from butane at reaction temperatures of 525.degree. to 600.degree. C. is only 25 to 52% (i.e., 14.8 to 30.8 mole % yield).
Harrison I, U.S. Pat. No. 3,915,892 also discloses an aqueous solution reduction method wherein a dihydrate catalyst precursor is formed which is subjected to rigidly controlled heating steps to form an anhydrous crystalline catalyst and bring about several phase transitions. This pretreatment procedure is complex and expensive and the maximum weight % yield from butane of maleic anhydride obtained at a reaction temperature of 465.degree. C. is only 87.3% (i.e., a mole % yield of 51.6).
Schneider, U.S. Pat. No. 3,864,280 discloses an organic solution reduction method wherein V.sub.2 O.sub.5 is dissolved in a V.sub.2 O.sub.5 /isobutanol slurry by passing a stream of anhydrous HCl gas into the slurry at a temperature of between 30.degree. and 40.degree. C. The resulting solution is then mixed with a solution of orthophosphoric acid in isobutanol and the resulting mixed solutions are heated to reflux; i.e., 110.degree. C., for 1.5 hours. The solvent is then evaporated and the resulting catalyst precursor is activated in air, and then in an air and butane mixture. The aforenoted process is conducted to impart to the catalyst a vanadium valence of plus 3.9 to 4.6, a P:V atomic ratio of 0.9 to 1.8:1, and an intrinsic surface area (i.e., the surface area of the catalyst in the absence of a support) of 7 to 50 m.sup.2 /gm. Optionally, the catalyst preparative method is conducted to impart a particular crystalline structure characterized as a B-Phase of at least 25%. To do this a minor amount of water must be present in the V-P-O isobutanol slurry used to prepare the catalyst thereby forming a hydrated precursor which looses its water of hydration upon activation bringing about the crystal phase change. The maximum weight % yield, obtained from butane at 370.degree. C. and after an unspecified reaction time, is disclosed as being 105% (i.e., 62.1 mole % yield).
Harrison II, U.S. Pat. No. 3,982,775 discloses an organic solution reduction method using HCl as well as an organic heterogeneous method wherein the vanadium and phosphorus containing components are reacted while suspended in an organic solvent. However, it is suggested therein that the solvent must contain at least about 20% by weight water. A dihydrate precursor is formed as in Harrison I and must be subjected to the complicated activation procedures disclosed therein. The dihydrate prepared by the non-aqueous heterogeneous method exhibits a DTA (differential thermal analysis) endothermic dip at 406.degree. C. The catalyst of Example 14 exhibits a weight % yield of maleic anhydride from butane at 418.degree. C. and after 160 hours of reaction of 102.6% (i.e., 60.7 mole % yield) the highest disclosed in this patent.
Katsumoto et al., U.S. Pat. No. 4,132,670 discloses an organic heterogeneous reduction method wherein V.sub.2 O.sub.5 is first partially reduced by refluxing in an organic media (e.g., isobutanol) for about 3 hours to reduce the average vanadium valence from +5 to about +4.5. Water formed during the reduction step may be removed by azeotropic distillation. A solution of orthophosphoric acid in isobutanol is then added to the reduced vanadium slurry and the vanadium and phosphorus components reacted at reflux temperature while removing water formed in-situ by azeotropic distillation. The resulting V-P-O suspended solids are removed from the organic medium by filtration. For fixed bed catalysts the solids are either pelleted and dried or extruded and dried. Extrusion can by achieved by adding sufficient water to the solid filter cake to form a paste, e.g., 1 part by weight water per 4.3 parts by weight solids. However, insufficient water is present during extrusion to alter catalyst properties and no extrusion temperatures are disclosed, it, therefore, being difficult to determine whether extrusion occurs at any temperature other than room temperature. Consequently, neither the amount of water nor the temperatures required to induce the catalyst properties obtainable by the water treatment step of the present invention are disclosed as being present during catalyst solids extrusion (see Example 1 therein). Furthermore, the catalyst of Katsumoto et al is calcined for fixed or fluid bed operations. This calcination is apparently conducted in accordance with the 2-stage activation procedure described at col. 7 lines 65 et seq. In the first stage the vanadium phosphate is heated in air at about 380.degree. C. (flow rate 2-3 v/v/min) for about 2 hours. In the second stage the air stream is then replaced by an air-butane mixture (1.5% by vol. n-butane) at a similar flow rate and temperature for about 15 hours. The temperature and flow rate are then adjusted to achieve a conversion of 90%. Thus, activation is always conducted by contact with air alone. In contrast, the performance of the catalyst of the present invention is actually significantly diminished if activated in air alone. Maximum mole % yields of maleic anhydride from butane disclosed in this patent are about 50% at about 425.degree. C. and 90% conversion.
Not all prior art methods employ strong acids for the purpose of reducing the average vanadium valence. For example, Hutchings et al., U.S. Pat. No. 4,209,423 (assigned to ICI Ltd.) discloses a method for preparing a V-P-O catalyst which has as its primary goal an increase in the proportion of a particular crystal phase in the catalyst designated as Phase-X and alleged to be primarily responsible for improved performance of the catalyst. The increase in Phase-X (subsequently designated in U.S. Pat. No. 4,222,945 as .alpha.-VPO.sub.5) is achieved by two essential procedures, namely, conditioning, by contacting, a catalyst precursor (i.e., the reaction product of a vanadium compound and a phosphorus compound) with an acid stronger than H.sub.3 PO.sub.4, e.g., HCl, to increase Phase-X directly, and by extracting at least one water soluble crystal phase, designated as Phase-E, to indirectly increase the proportion of Phase-X by removing non-Phase-X portions of the catalyst. Thus, in one embodiment a vanadium compound, e.g., V.sub.2 O.sub.5 is dissolved in a concentrated aqueous acid, e.g., HCl solvent. To this solution is added a phosphorus compound, such as orthophosphoric acid, which reacts with the vanadium compound to form a vanadium/phosphorus mixed oxide catalyst precursor. Alternatively, the V.sub.2 O.sub.5 and orthophosphoric acid can be dissolved initially in the same aqueous HCl solvent, and reacted in the same pot. As a further alternative, a compound of vanadium and phosphoric acid can be reacted in the presence of water and/or a lower alcohol, e.g., methanol, (water alone being the preferred solvent) to produce a form of alpha VOPO.sub.4 which is then conditioned in a solution of a strong acid to form the precursor. In all instances, a strong acid conditioning step is employed, and the conditioned precursor must be removed from an aqueous acid solution (unlike the process of the present invention which forms a heterogeneous suspension of the first catalyst precursor), e.g., by evaporation of the acid solvent. The dry or nearly dry conditioned precursor is then extracted with water or another solvent, [e.g., by boiling in water (20 ml/g solid) for 3 hours, filtered hot, washed in warm water and dried in air at 60.degree. C.] to remove a crystal phase from the precursor identified as VO(H.sub.2 PO.sub.4).sub.2 and designated Phase-E as described above. The precursor is then calcined in an air and butane mixture at 385.degree. C. for 100 hours. Other properties possessed by the catalyst in addition to Phase-X, are the additional presence of Phase-B as defined in the Schneider Patent discussed above, a P:V atomic ratio of 1:0.5 to 1.2:1, and a surface area of at least 10 m.sup.2 /g. No mention of vanadium valence is made in this patent as being critical to catalyst performance. The maximum mole % yield of maleic anhydride from butane using an unpromoted catalyst is disclosed as being 53%, at 385.degree. C. for an aqueous solution method (i.e. Example 3). However, in Example 7 when V.sub.2 O.sub.5 and orthophosphoric acid are reacted in methanol (by refluxing), contacting the precursor with aqueous HCl, extracting the precursor with water, and calcining, the mole % yield is only 44% at a reaction temperature of 385.degree. C. and 47% at a reaction temperature of 420.degree. C.
Higgins et al., U.S. Pat. No. 4,222,945 (also assigned to ICI) describes a process similar to Hutchings et al, except that the mean crystallite size of the water extracted precursor is controlled within defined limits by ball milling the water extracted precursor in the presence of a solvent such as cyclohexane, preferably in the presence of a dispersant. The resulting catalyst is used to oxidize a hydrocarbon-air mixture containing at least 10 mole % hydrocarbon. Maximum mole % yield of maleic anhydride from butane is about 10%.
Generalizing from the above discussion, conventional preparative methods, including both the aqueous and organic solution techniques, are unsatisfactory in that:
(1) they usually require that the catalyst manufacturing equipment be fabricated of special corrosion-resistant materials of construction;
(2) they are troubled by serious waste-disposal problems arising out of the employment of hydrogen chloride, nitric acid or oxalic acid for the dissolution of the vanadium component;
(3) they generally require extended and complex procedures for activation of the precursor catalyst;
(4) the preparation of the precursor catalyst is generally complicated and inherently costly; and
(5) the aqueous-based preparations result in catalysts of relatively poor activity and yield for converting butane to maleic anhydride.
The organic heterogeneous non-HCl method of Katsumoto et al. simplifies the preparative procedures of Schneider but at the expense of drastically reduced yields. The V-P-O preparative method and water extraction technique as described and practiced in Hutchings et al. also results in drastically reduced yields.
The known mixed oxide compositions for the catalytic conversion of hydrocarbons to carboxylic acid anhydrides also suffer from a number of disadvantages which include relatively poor selectivities, poor activities at low operating temperatures, poor stability manifested by short operational lifetimes, inadequate yields and a required activated procedure which is long and complicated.
Accordingly, there has been a continuing search for new and improved V-P-O containing catalysts and methods of their preparation which produce higher yields than heretofore obtainable in the prior art. The present invention is a result of this search.